1. Field of the Invention
This invention generally describes new methods and apparatus to detect and locate devices underwater. Specifically, this invention relates to an underwater location system comprising an underwater beacon apparatus which emits signals and a receiver apparatus which is capable of detecting the beacon signals and locating the underwater beacon.
2. Prior Art
Conventional underwater location devices, or pingers, emit periodic acoustic pulses. Hand-held, towed, or ship-mounted underwater location device (ULD) receivers are used to locate the pinger's position. The pulsed signal has limited range, is subject to distortion, and is difficult to locate using the existing receivers. Historically, the major use of underwater location devices (ULDs), has been for aircraft flight recorders. The Federal Aviation Agency (FAA) established the rules, FAR 25.1457(g)(3), 25.1459(d)(3), 121.343(f), and 121.359(c)(2)(iii), that cockpit voice and flight data recorders carried aboard most commercial aircraft must have an approved underwater location device attached. To be an acceptable ULD, an acoustic beacon must meet the specifications outlined in FAA Advisory Circular AC-21-10A. These standards were developed and are continually reviewed by a special committee of the Society of Automotive Engineers (SAE - Committee A-4). Advisory Circular AC-21-10A specifies that the ULD must emit a repeating acoustic pulse. The pulse rate is approximately one pulse per second. The beacon, once activated by water immersion, must operate continuously for a minimum of 30 days. Experience during past recovery operations has shown the range of the current conventional ULD design to be approximately one mile.
Unfortunately, even with acoustic beacons attached, underwater objects are not always located when situated in large bodies of water. Recent examples include KAL Flight 007, a widebodied aircraft, which plunged into the Sea of Japan on the night of Aug. 31, 1983. The flight data recorders were never recovered from this highly-publicized incident. The flight data and cockpit voice recorders were eventually recovered from the wreckage of Air India's Flight 182, a Boeing 747, that apparently exploded over the North Atlantic on June 23, 1985. Although the crash site of the Air India plane was localized to a five-square-mile area, it took searchers, representing the efforts of six nations, over two weeks to locate the recorders. The recorders were found at a depth of 6,700 feet. In 1974, TWA's Flight 841 was sabotaged over the Ionian Sea. A nearby aircrew witnessed Flight 841's rapid descent and disintegration. This eye-witness account allowed an accurate position-estimate of the crash site. Although the flight recorder contained an acoustic beacon and the crash site was known, it still took 27 days of searching by the U.S. Sixth Fleet to locate the beacon's acoustic signal. Note that the operating life of the beacon may be only 30 days. The conventional type of pinger which is used on flight recorders offers no security against unauthorized location or intentional jamming efforts such as may have happened at the KAL accident site.
Pingers are also used to locate torpedoes in underwater test and practice ranges. Before being delivered to the Navy, every new torpedo is test-fired at least once at a practice range. Sometimes these torpedoes are lost during the course of a test. This is a serious problem. Recovery of an advanced torpedo by a foreign government not only compromises the technology in the weapon, but can render an entire weapon system susceptible to foreign countermeasures. The search and recovery operations used to retrieve these torpedoes can be expensive and time-consuming. Underwater location devices are often placed on the torpedoes for the practice shots. The ULDs can greatly assist the location efforts.
Two common uses of ULDs have been described. Pingers have been employed in other Government and commercial applications. These include:
Space Nuclear Power Systems During Launch PA0 Other Space Launches, i.e. Space Shuttle & Cargo PA0 Classified Material Transport PA0 Military Salvage Operations PA0 Nuclear Weapon Transport PA0 Military Flight Recorders PA0 Missile Test Firings PA0 Naval Mine Operations PA0 Downed Airman Beacons PA0 Military Aircraft PA0 Commercial Space Payloads PA0 Marine Salvage PA0 Ocean Research PA0 Marine Mammal Studies
Commercial uses include:
Underwater acoustic pingers are manufactured by a variety of U.S. companies. The Dukane Corporation's Seacom Division, located in St. Charles, Ill., is a dominant company in the world market. Other manufacturers include Data Sonics, Inc., Sonatech, Inc., and EFCOM Subsea Technology.
Most pingers are cylindrical in shape. Typical dimensions for flight recorder applications are 1.25 inches in diameter and 3.8 inches long. The flight recorder ULDs can survive crash forces of 6000 g's (0.5 ms) and ocean depths of 20,000 feet. A small contact pad on one end of a pinger is shorted by salt or fresh water contact. This activates the beacon. Including the battery, the device weighs approximately 9 ounces.
The conventional pingers, which are attached to flight recorders, emit a 9 millisecond pulse of a 37.5 Khz carrier. The pulse is repeated every 1.1 seconds. The pinger's output is a finite-lengthened wavetrain. SAE Aerospace Standard AS-8045 stipulates that the minimum acoustic source strength of location pingers must be 160.5 dB (re:1 micro-Pa). Different pinger strengths, frequencies and pulse lengths are also used in the other pinger applications.
As the signal radiates from a submerged underwater beacon, energy is lost due to both geometrical divergence (spreading) and the intrinsic attenuation of seawater. At the receiver, the signal is detected along with the background acoustic noise of the ocean.
If the ocean were noise free, it would be possible to detect the pinger's signal at great ranges. Likewise, if the underwater beacon had no power restrictions, a large source strength could be used to overcome ocean noise interference. However, because the oceans are noisy and the ULDs are power limited, it is generally not possible to detect isolated pulses over ranges greater than approximately one mile. The basic problem is that the signal level rapidly falls below the ocean noise level, and the pulses become undetectable.
Problems remain and necessities exists to improve detectability of pinger signals and to provide longer detection ranges. An improved detection range would mean quicker locations, lower-cost search operations, and a higher probability-of-detection. In airplane accident investigations, recovery of the flight recorders is often essential for establishing the cause of the accident. When large areas must be searched, heretofore unattainable longer detection ranges are critical. Longer operating times then currently available may be extremely important.